1. Field of the Invention
The present invention relates to a powdery material having a negative coefficient of linear thermal expansion and an improved low expansible sealing composition containing the same. More particularly, the present invention relates to a composition for sealing IC packages, etc.
2. Description of the Prior Art
Conventional sealing compositions comprising soldering glass as the main component are used for sealing, bonding or coating previously molded articles, glassware, metal elements, ceramics, semiconductor elements or portions of semi-conductive components, etc. The properties required of sealing compositions used for this purpose are low expansivity and high electrical insulating property, ability to form a firm seal and suitable fluidity. For example, the sealing glass used for sealing IC packages or display tubes or for furnace sealing of Braun tubes must have a coefficient of linear thermal expansion lower than that of Al.sub.2 O.sub.3 and higher than that of Kovar, a glass transition point of 400.degree. C. or less and a good electrical insulating property, because it is used to seal ceramic base plates comprising a low expansible metal such as Kovar, and Al.sub.2 O.sub.3 as main components. However, the low melting glasses generally used for this purpose have a high coefficient of linear thermal expansion.
In order to overcome this fault, it has been attempted to carry out a process for forming crystals of lead titanate or .beta.-eucryptite by heat treatment of, for example, a Li.sub.2 O-Al.sub.2 O.sub.3 -SiO system or a PbO-B.sub.2 O.sub.3 -TiO.sub.2 system crystalline glass. However, in the former glass, the electrical insulating property of the seal deteriorates due to the presence of Li ions and the properties of the coated or sealed semiconductor elements deteriorate upon diffusion of Li ions. On the other hand, with the latter glass the sealing treatment requires a comparatively long period of time due to its high sealing temperature.
In addition, a low expansible filler such as zircon, quartz, .beta.-eucryptite, etc., has been added to a powder of a PbO-B.sub.2 O.sub.3 -SiO.sub.2 system or a PbO-ZnO-B.sub.2 O.sub.3 system crystalline glass in order to reduce the coefficient of linear thermal expansion as disclosed in U.S Pat. Nos. 3,250,631 and 3,258,350. Taking zircon as an example, it has good acid resistance, but large additions are necessary to reduce the coefficient of linear thermal expansion to a desirable level since it has a coefficient of linear thermal expansion of about 45.times.10.sup.-7 /.degree.C. and, consequently this makes the sealing temperature high. As another example in addition to the above described fillers, lead titanate has been used to reduce the coefficient of expansion. Lead titanate has a coefficient of linear thermal expansion of -60.times.10.sup.-7 /.degree.C. and exhibits negative expansion. The fluidity of the glass to which it is added is good but it has a high dielectric constant.
.beta.-Eucryptite has a coefficient of linear thermal expansion as low as -120.times.10.sup.-7 /.degree.C. and, consequently, it has an effect of reducing the coefficient of linear thermal expansion when used in a small amount. Further it has a low dielectric constant, but when added to glass, it dissolves in the glass and deteriorates the fluidity. Furthermore, the Li.sub.2 O contained in .beta.-eucryptite diffuses into the glass and deteriorates its electrical insulating property.